A PLL conventionally comprises a voltage controlled oscillator (VCO) delivering a signal at high frequency, a frequency divider (also called loop divider) converting the signal at high frequency into a signal at divided frequency, a phase comparator producing a measurement signal for measuring a phase deviation between the signal at divided frequency and a reference signal at a reference frequency, and a low-pass filter (also called loop filter) to which is applied the measurement signal and whose output controls the VCO.
In the application to frequency synthesis, the value of the division factor applied by the frequency divider is adapted so as to obtain a preset value for the frequency of the output signal of the VCO. Specifically, we have the following relation:Fout=N×Fref  (1)
where Fout is the frequency of the output signal of the VCO;
where Fref is the reference frequency of the PLL; and,
where N is the (integer) division support of the loop divider.
The passband of the PLL corresponds to that of a low-pass filter. It determines the speed of the response to a modification of the preset. It is a significant element in the performance of the synthesizer incorporating this PLL. Specifically, the larger the passband of the PLL, the faster the stabilization of the synthesized frequency, upon a change of radio channel, for example.
Moreover, the passband dictates the monitoring of the phase noise of the source of the reference signal (which is often better than the phase noise of the VCO at low frequencies) in this frequency band. Consequently, the larger the passband, the weaker the phase noise of the PLL, and hence of the synthesizer, at low frequencies.
This is why it is desirable to widen the passband of the PLL, so as to optimize the speed and reduce the phase noise of the frequency synthesizer incorporating it.
The passband of the PLL is essentially determined by the passband of the loop filter and by the open-loop gain of the PLL. Now, the passband of the loop filter is chosen so as to filter the spurious line generated by the phase comparator at the reference frequency.
To ensure the stability of the loop, it is advisable to take for the loop filter a passband equal to or less than
      Fref    10    ,or even
      Fref    15    .
Compliance with this criterion ensures good stability and good rejection of the spurious line at Fref generated by the phase comparator.
The passband of the PLL therefore depends directly on the reference frequency Fref of the PLL. The higher the frequency Fref, the wider this passband. But the reference frequency also defines the synthesis interval, that is to say, for example, the frequency gap separating two adjacent radio channels (for example, 200 kHz for GSM). The passband of the PLL therefore depends on the reference frequency, which itself depends on the standards envisaged (by way of the synthesis interval).
In the prior art, a solution for increasing Fref without impairing the synthesis interval has already been proposed. This is the fractional (i.e. noninteger) PLL.
The fractional PLL has established itself as a compromise, by replacing the integer division ratio frequency divider with a fractional frequency divider (noninteger division). It is thus possible to increase the reference frequency (for example from 200 kHz to 10 MHz) while retaining the same synthesis interval (200 kHz in the example).
A fractional PLL uses a fractional loop divider, based on the use of a Sigma-Delta modulator whose output drives the control input for the division ratio of the divider. Such a fractional divider divides by N during a determined number P−1 of cycles of the reference signal and by N+1 during a cycle of the reference signal. We thus obtain, on average, the following relation:
                    Fout        =                  Fref          ×                      (                          N              +                              1                P                                      )                                              (        2        )            
The synthesis interval becomes less than the reference frequency, since it is equal to about
  Fref  ×            (              1                  P          2                    )        .  
For a identical synthesis interval, it is thus possible to use a higher reference frequency, thereby also making it possible to increase the passband of the PLL.
This frequency is however obtained solely as an average, over N periods of the reference signal, this having the direct consequence of causing spurious lines to appear at output. These spurious lines limit the utilizable passband of the fractional PLL. Admittedly, the Sigma-Delta modulator shapes the noise in the high frequencies, but we are nevertheless compelled to lower the cutoff frequency of the loop filter to filter it.
For this reason, the fractional PLL is not entirely satisfactory from the point of view of the problem posed.